drop size dependence of contact angles and

216 Pages·2001·11.06 MB·English

DROP SIZE DEPENDENCE OF CONTACT ANGLES AND
LINE TENSION
A thesis subrnitted in conformity with the requirernents for the degree of
Doctor of Philosophy
Graduate Department of Mechanicd and InduStnaI Engineering
in the University of Toronto
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DROP SIZE DEPENDENCE OF CONTACT ANGLES
AND LINE TENSION
Doctor of Philosophy, ZOO 1
Alidad Arnirf'mli
Department of Mechanical and Industrial Engineering
University of Toronto
Abstract
The generalized theory of capillarity identifies line tension as a natural and necessary
extension of analysis for the systems with multi-phase boundary curves. Theoretically,
finding an estirnate for line tension is very complicated. Modeling of molecular interactions
almost always requires significant simplifications which would cause considerable
uncertalnty in the resulting values. Experimentally, line tension is typically small and hence
rnuch more difficult to measure than surface tension. A review of the Iiterature shows that
there is little consensus among researchers with respect to both sign and magnitude of Iine
tension.
Our preferred method to detennine line tension for solid-liquid-vapor systems takes
advantage of dependence of contact angles on the drop size according to the modified
Young equation (Eq. 2.22). Using a sensitive and accurate contact angle and drop size
rneasurement methodology, Le., Axisymmetnc Drop Shape Analysis (ADSA), a large
number of contact angle data were pmduced Le., 27, solid-liquid-vapor systems were
studied. Potentid artifacts that may effect the observed drop size dependence of contact
angles including contortions of the three-phase line, solid surface deformation, thin film
effect, and heterogeneity of the solid surface, were considered. They all proved hsignifÏcant
either by the use of models developed to descnie and analyze the effect of the particula.
feature, or by thermodynamic reasoning, or the use of available literature (see section 4.4). It
was established that line tension was positive for al1 of the systems shidied. The magnitude
of line tension detemiined ranges from below fiorri 1 0 J~/m~ f or systems with low contact
angles to IO-' J/m For the high energy system studied (the range reflects mainly the matenal
properties such as intemolecular forces). Exploratory steps taken to examine line tension
behavior near the wetting transition suggested that the h e t ension decreases as the contact
angle decreases and perhaps vanishes at complete wetting. A high energy system, Le., liquid
th on a silica surface, was also studied. The seemingly large iine tension value obtained, Le.,
104 Um, should not be entirely surprising, as it was found that there exists a positive
correlation between line tension and solid-liquid interfacial tension.
1 would like to thank Professor A.W. Neumann for his continued guidance, and support at
the key moments throughout the course of this thesis. I leamed not ody a great deal about
surface thermodynamics Eom him, but also he was a role model in professionalism. 1 must
also thank Professor Neumann for taking the the to ensure that 1 could effectively
communicate my ideas, pdcularly on paper.
1 also would like to thank al1 my colleagues Eorn past and present, and the
undergraduate students under my supe~sion,a t the Laboratory of Applied Sutface
Themodynamics, for their assistance and advice. Assistance of Ms. 2. Policova in
experimental matters is also acknowledged.

Chapter 3 .E WENMENTAL METHODS A DP ROCEDURE ............................. 72
............................................................
3.1 Contact Angle Measurements 72
3. la Axisymmetnc Drop Shape Analysis-Profile (ADSA-P) ........................ 76
..................................................................................
3 Theory 76
P Experimental Procedure and Apparatus ............................................ 81
....................
3.1 b Axisyrnmetric Drop Shape Analysis-Diameter (ADSA-D) 85
..................................................................................
3 Theory 86
.............................................
î Experimental Procedure and Apparatus 87
............
3.2 Cleanuig Procedure for Glassware and Test Liquid Handling Material 91
.....................................................................................
3.3 Materials 92
...........................................
3.4 Solid Surface Requirements and Preparation 96
3.4a Preparation Procedure for Alkanethiol SAM Surfaces on Gold ................ 97
> ...................................................................
Substrate Fabrication 97
> Monolayer Preparation ................................................................ 102
3.4b Roughness and Homogeneity Characteristics of SAM Surfaces ............... 103
..................................................................
3.5 Micropipette Cdibration 110
........................... ... ..............**..*....
3.6 Evaporation Rate Measurements 114
Chapter 4 .R ESUL TS AND DISCUSSIONS ....................................................1 17
4.1 Drop Size Dependence of Contact Angles for Low Energy Systems .............. 117
....................
4.la Experimental Results for the Single Species SAM Surfaces 122
4.1 b Experimental Results for the Dual Species (Mixed) SAM Surfaces .......... 129
4.2 Drop Size Dependence of Contact Angles for Systems with Low Contact Angles 136
4.3 Drop Size Dependence of Contact Angles for a High Energy System .............. 141
4.4 Exclusion of Causes Other Than Line Tension for the Observed Drop Size
Dependence of Contact Angles ......................................................... 145
4.4a Viscous Effects ...................................................................... 146
4.4b Solid Surface Deformation ......................................................... 146
4.4~T hin Film Effects .................................................................... 147
...............................................................
4.4d Heterogeneity Effects 148
..................................................................
4.4e Evaporation Effects 152
4Af Hydrostatic Pressure EEects .............................. ... ..................... 155
.................
4.5 Correlation of Line Tension with Solid-Liquid Interfacial Tension 157
Chapter 5-SUMMARYm CONCLUSIONSFUTURE WORK ........................ 165
Chapter 6 -FUTLIRE WORK .................................................................... 169
.................................~........................................................
References 172
Appendir A .S andwich Drop Method and Surface Preparation for High Energy System 18 5
A.1 Sandwich Drop Technique .............................. .. ........**.................1 85
A.2 Experimental Procedue and Apparatus ............................... . .......... 186
A.3 Surface Preparation for High Energy S ystem ......................................... 187
Appendix B Patterning a SAM Surfaces Using a LASER Beam .......................... 190
w
................................................................................
B 1 o d c t i n 190
............................ .. .......................
B.2 Description of the Method 191
B.3 Results ...................,...................,,.,.................. ........................... 192
Appendir C .S oftware Application to Extract Contact Line Profile of a Meniscus in
Contact with a Heterogeneous Wall Composed of Altemating
..........................................
Hydrophobic and Hydrophilic Stips 198
List of Tables
TABLE 3.1 Physical properties of test liquids; dl data provided by supplier, except surface
tension,
TABLE 3.2 Summary of substrate preparation procedure.
TABLE 4.1 Liquids studied, number of solid surfaces (methyl temiinated SAM, 1-
octadecane thiol) used for each liquid, and the total number of contact angle
measurements for each system (experiments were performed at 22+ 1O C).
TABLE 4.2 Summary of experimental results obtained for 8 liquids on methyl terminated,
1-octadecanethiol, SAM surface (experiments were performed at 2211°C).
TABLE 4.3 Liquids studied, total nurnber of contact angle measurements, correlation
coefficient (r) and its significance level for the studied systems on the mixed
SAM d a c eo f [CH3 (88%) - COOH (12%)] (experiments were performed at
22k1°C).
TABLE 4.4 Liquids studied, total number of contact angle measurements, correlation
coeficient (r) and its significance level for the studied systems on the mixed
-
SAM surface of [CH3 (80%) COOH (20%)] (experiments were performed at
2211°C).
TABLE 4.5 Surnmary of experimentd results: line tension, Q,, and correlation coefficient,
r, for pure methyl terminated SAM surfaces (experiments were performed at 23
OC).
TABLE 4.6 Summary of experimental results: line tension, O ,, and correlation coefficient,
-
r, for SAM-1 surfaces [CH3 (88%) COOH (12%)] (experiments were
performed at 21 OC).
TABLE 4.7 Surnmary of contact angle (0,) and line tension, o, for Iow contact angle
systems,
TABLE 4.8 Modei calculations for the effect of three-phase line contortions on the drop
size dependence of contact angles for several different mode1 heterogeneous
surfaces,
TABLE 4.9 Summary of the average evaporation rate results. The experiments were
performed rmder normal laboratory conditions in open air at 23 OC. The
experiments were conducted in a corner of the laboratory where air currents
were deemed to me minimal. At least three similar drops were used to obtain
the average evaporation rates for each system.
TABLE 4.10 Summary of calculated hydrostatic, Laplace, and total pressures inside of
water droplets (at the three-phase Line)of various size placed on a methyl
terminated SAM surface,
TABLE 4.1 1 Summary of cdculated hydrostatic, Laplace, and to ta1 pressures inside of
nonane droplets (at the three-phase he)of various size placed on a methyl
terminated SAM dace.
TABLE 4.12 Summary of experimental results for Line tension and solid-liquid interfacial
tension fiom al1 systems studied in this thesis.